In a circuit which amplifies or latches a broadband high-frequency signal, in many cases, differential signals including a pair of signals Vin(+) and Vin(−) whose poralities are inverted each other are used. A differential circuit which amplifies or latches the differential signals includes differential input terminals or differential output terminals as a pair of differential terminals.
As such, when the differential circuit which transmit or receive the differential signals is formed on a semiconductor substrate by an integrated circuit (IC) technique, a differential distributed amplifier or a distributed logic circuit has been proposed as a method for obtaining broadband characteristics (Patent Documents 1 and 2).
These circuits have a structure in which the input and output terminals of a plurality of differential circuits that are arranged at regular intervals are connected in series to each other by a transmission line with high characteristic impedance. An artificial transmission line with a high cutoff frequency is formed by the input and output capacitances of the differential circuits and the inductance component of the transmission line. Therefore, it is possible to transmit the differential signals to a plurality of differential circuits in a wide band.
FIG. 16 is a diagram schematically illustrating an example of the layout of a differential distributed amplifier which is formed on a semiconductor substrate, in which a plurality of (in this example, four) differential amplifier circuits 1(1) to 1(4) are arranged in a line at predetermined intervals along the X-axis on a semiconductor substrate (not shown) made of, for example, InP (indium phosphorus) or GaAs (gallium arsenic), with the input and output terminals of all circuits aligned in the same direction. A differential signal transmission line 10 for inputting a signal to a differential amplifier circuit is formed along the X-axis so as to face a pair of differential input terminals 1a and 1b of each differential amplifier circuit and a differential signal transmission line 90 for outputting a signal from the differential amplifier circuit is formed along the X-axis so as to face a pair of differential output terminals 1c and 1d of each differential amplifier circuit.
The differential signal transmission line 10 includes a pair of main lines 11 and 15 which extend in parallel along the X-axis with a predetermined gap G therebetween. The main line 11 includes main line conductors 11b to 11f which are connected in series to each other and the main line 15 includes main line conductors 15b to 15f which are connected in series to each other. The main line conductor is a strip-shaped conductor (formed from metallic, e.g. gold alloy) whose characteristic impedance is arbitrarily set and is represented by a rectangle. A line connecting the main line conductors indicates the connection between the main line conductors. In addition, a branch point of the line is represented by a black circle and a terminal is represented by a white circle.
Differential signals Vin(+) and Vin(−) which are input to start terminals 11a and 15a of the main lines 11 and 15 are absorbed by termination resistors 14 and 18 connected to end terminals 11g and 15g of the main lines 11 and 15. The main lines 11 and 15 are connected to the differential input terminals 1a and 1b of the differential amplifier circuits 1(1) to 1(4) by branch lines 13a to 13d and 17a to 17d which extend along the Y-axis and have a predetermined length. The branch lines 17a to 17d and the main line conductors 11c to 11f intersect each other, with an insulating layer interposed therebetween.
Each of the main line conductors 11b to 11f and 15b to 15f is a transmission line with high characteristic impedance. For example, FIG. 17 shows an equivalent circuit of the structure in which each of the main line conductors 11b to 11f is connected to the differential input terminal 1a of each differential amplifier circuit. The inductance component L of the main line conductor and the parasitic capacitance C of the input terminal 1a form an artificial transmission line. The circuit is designed such that the inductance L and the capacitance C are optimized to obtain broadband transmission characteristics with a high cutoff frequency. In this case, the characteristic impedance of the artificial transmission line is set to be equal to the resistance value of the termination resistor 14.
In FIG. 16 which is a schematic diagram, the differential signal transmission line 90 has a structure which is reversed to the differential signal transmission line 10 which is on the differential input terminal side on the substrate (is obtained by rotating the differential signal transmission line 10 by 180 degrees). Differential output terminals 1c and 1d of each differential amplifier circuit are connected to the differential signal transmission line 90. In the differential distributed amplifier, the output signals which have been input from the differential signal transmission line 10 and then amplified by the differential amplifier circuits are combined with same phase on the differential signal transmission line 90 and signals Vout(+) and Vout(−), which are the amplified signals of the original input signals Vin(+) and Vin(−), are output.
The differential signal transmission line 90 includes a pair of main lines 91 and 95 which extend in parallel along the X-axis with a predetermined gap G′ therebetween. The main line 91 includes main line conductors 91b to 91f which are connected in series to each other from a start terminal (output terminal of the circuit) 91a and the main line 95 includes main line conductors 95b to 95f which are connected in series to each other from a start terminal 95a. The main lines 91 and 95 are connected to differential output terminals 1d and 1c of each differential amplifier circuit by branch lines 93a to 93d and 97a to 97d which extend along the Y-axis and have a predetermined length. The branch lines 97a to 97d and the main line conductors 91c to 91f intersect each other with an insulating layer interposed therebetween. The ends of the main line conductors 91f and 95f are connected to termination resistors 94 and 98, respectively.
Similarly to the input line, the output line is designed such that the inductance components of the main line conductors 91b to 91f and 95b to 95f and the parasitic capacitance of the differential output terminals 1c and 1d form an artificial transmission line with broadband characteristics. In this case, the characteristic impedance of the artificial transmission line is set to be equal to the resistance value of the termination resistor 94 or termination resistor 98.
The electrical lengths of the differential signal transmission lines 10 and 90 are optimized such that the signals input from the start terminals of the differential signal transmission line 10 are sequentially amplified by the differential amplifier circuits 1(1) to 1(4) while being propagated to the end terminals and the amplified signals are combined with same phase while being propagated to the start terminals (the output terminals of the circuit) of the differential signal transmission line 90.
As such, the differential distributed amplifier combines the output signals amplified by a plurality of differential amplifier circuits with same phase. Therefore, it is possible to amplify broadband differential signals with a high gain.